Controller for power converter

ABSTRACT

A controller for a power converter detects a current output from the power converter, estimates a flux vector of the motor, and calculates a torque line in a physical model resulting from mathematizing a circuit equivalent to the motor, based on the estimated flux vector. The torque line indicates a line for obtaining a desired torque in a subsequent control period. The controller calculates a stator flux command value in accordance with a loss, calculates a voltage command value based on the torque line and the stator flux command value, and controls an output voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An embodiment described herein relates generally to a controller for apower converter that drives a motor.

2. Description of the Related Art

In general, a regeneration brake is known as means for decelerating amotor. When the regeneration brake is used, it must consume theregeneration energy generated by the motor. As a method of consuming theregeneration energy, there is a method of, for example, providing aresistance on the DC-side of an inverter for consuming the energy. Inthis case, it is necessary to add a resistor as new hardware. There isanother method, in which the energy is returned to the power supplyside. In this method, a converter for supplying DC power to the inverterthat drives the motor is required to perform inversion, and hence it isnecessary to replace a diode rectifier with an expensive converter thanthat. In light of these requirements, such methods as below utilizingcontrol by an inverter have been proposed.

A first method is a method of strengthening, utilizing indirect fieldoriented control (IFOC), a magnetic flux within a range in which the DCcapacitor voltage does not exceed the upper limit, thereby causing amotor to consume energy (Marko Hinkkanen, et al., “Braking Scheme forVector-Controlled Induction Motor Drives Equipped With Diode RectifierWithout Braking Resistor,” IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS,IEEE, September/October 2006, VOL. 42, No. 5, pp. 1257-1263).

A second method is a method of superimposing different frequencies, andis also a braking method utilizing the above-mentioned IFOC. In thismethod, a relatively low-order frequency current is superimposed on anoutput current to cause a motor to consume energy (Mukul Rastogi, etal., “Dual-Frequency Braking in AC Drives,” IEEE TRANSACTIONS ON POWERELECTRONICS, IEEE, 2002, November, VOL. 17, No. 6, pp. 1032-1040).

A third method is a high-frequency wave superimposing method, and isalso a braking method utilizing the above-mentioned IFOC. In thismethod, a current containing a large number of high-frequency componentsis superimposed on an output current to thereby cause a motor to consumeenergy (Jinsheng Jiang, et al., “An Efficient Braking Method forControlled AC Drives With a Diode Rectifier Front End,” IEEETRANSACTIONS ON INDUSTRY APPLICATIONS, IEEE, 2001, September/October,VOL. 37, No. 5, pp. 1299-1307).

IFOC is a motor control method known as general vector control. However,the above-mentioned methods principally have the following problems.Since IFOC is a method of controlling current to indirectly controlflux, and the first-order lag transfer function determined from a motorconstant exists between the current and the flux. Accordingly, a motorresponse beyond the first order lag cannot be expected. Further, in thesecond and third methods, frequencies other than the inverter outputfrequency are superimposed, which principally causes torque ripple.

In contrast to the above three methods, there is a direct torque controlmethod (fourth method). The fourth method is a method of directlycontrolling torque and flux to increase the magnitude of the stator fluxwith a fast response so as to make a motor consume energy. In thiscontrol, since the flux can be directly controlled, there is noinfluence of the first order lag. Therefore, the fourth method exhibitsa high response compared to IFOC. In the fourth method, however, inorder to determine a switching pattern on the inverter for driving themotor, hysteresis must be provided for the torque and flux control,which principally causes torque ripple (Cristian Lascu, et al., “AModified Direct Torque Control for Induction, Motor Sensorless Drive,”IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, IEEE, January/February 2000,VOL. 36, No. 1, pp. 122-130).

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a controller for a powerconverter, the controller capable of putting, on a motor, a brake ofgood response with no torque ripple.

In accordance with an aspect of embodiments, there is provided acontroller for a power converter configured to drive a motor. Thecontroller comprises a current detector configured to detect a currentoutput from the power converter; a flux vector estimation moduleconfigured to estimate a flux vector of the motor based on the currentdetected by the current detector; a torque line calculation moduleconfigured to calculate a torque line in a physical model resulting frommathemating a circuit equivalent to the motor based on the flux vectorestimated by the flux vector estimation module, the torque lineindicating a line for obtaining a desired torque in a subsequent controlperiod; a stator flux command value calculation module configured tocalculate a stator flux command value in accordance with a loss; avoltage command value calculation module configured to calculate avoltage command value based on the torque line calculated by the torqueline calculation module and the stator flux command value calculated bythe stator flux command value calculation module; and an output voltagecontroller configured to control an output voltage of the powerconverter based on the voltage command value calculated by the voltagecommand value calculation module.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a block diagram showing the configuration of a motor drivesystem according to an embodiment;

FIG. 2 is a conceptual diagram according to the embodiment showing, in asingle voltage-second coordinate system, the torque line derived from aphysical model of an induction motor, the range of a stator flux thatcan be output in a subsequent control period, and a loss model;

FIG. 3 is a graph according to the embodiment analytically showing themagnitude range of the stator flux of the induction motor;

FIG. 4 is a graph showing plots of speed against time obtained whenbraking control is performed by the controller according to theembodiment;

FIG. 5 is a graph showing plots of flux command values against timeobtained when braking control is performed by a controller according tothe embodiment;

FIG. 6 is a graph showing plots of torque against time obtained whenbraking control is performed by the controller according to theembodiment; and

FIG. 7 is a graph showing plots of DC voltage against time obtained whenbraking control is performed by the controller according to theembodiment.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 1 is a block diagram showing the configuration of a motor drivesystem 10 according to an embodiment. In the accompanying drawings, likereference numbers denote like elements, and different elements will bemainly described in detail.

The motor drive system 10 includes a controller 1, an AC power supply 2,a diode rectifier 3, a capacitor 4, an inverter 5, an induction motor 6,current detectors 7 a, 7 b and 7 c of three phases, and a phase sensor8. The motor drive system 10 operates a load machine 9 using theinduction motor 6 as a mechanical power source.

The AC power supply 2 supplies three-phase AC power to the dioderectifier 3. For instance, the AC power supply 2 is a grid.

The diode rectifier 3 converts, into DC power, the three-phase AC powersupplied from the AC power supply 2. The power supply is not limited tothe diode rectifier 3, but may be any other power supply, if it suppliesa DC power to the inverter 5. However, power is not regenerated to theAC power supply 2, and hence the power supply may be aregeneration-disabled one.

The capacitor 4 is provided across a DC link that connects the dioderectifier 3 to the inverter 5. The capacitor 4 is used to smooth the DCvoltage (DC link voltage) applied to the DC link.

The inverter 5 converts the DC power supplied from the diode rectifier 3into three-phase AC power for driving the induction motor 6. Theinverter 5 is a voltage source inverter. The inverter 5 is subjected tovariable voltage variable frequency (VVVF) control performed by thecontroller 1 through pulse width modulation (PWM) control. The powerconversion circuit of the inverter 5 is formed of three arms. One of thearms includes an upper arm and a lower arm. The upper and lower arm areeach formed of at least one switching element.

The induction motor 6 is driven by the three-phase AC power output fromthe inverter 5.

The current detectors 7 a, 7 b and 7 c of three phases are provided atthe respective phases of the output side of the inverter 5. The currentdetectors 7 a, 7 b and 7 c detect the stator currents Ias, Ibs, Ics ofthe three phases, respectively. The current detectors 7 a, 7 b and 7 coutput the detected three-phase stator currents Ias, Ibs, Ics to thecontroller 1. The stator currents Ias, Ibs, Ics are those output fromthe inverter 5 and input to the stators of the induction motor 6.Further, it should be noted that if currents of the two phases aredetected, the remaining one-phase current can be calculated.Accordingly, two current detectors may be provided for the two phases.

The phase sensor 8 detects the signal output in accordance with therotation of the rotor of the induction motor 6 in order to estimate therotor angle (electric angle) θr(k) of the induction motor 6. The phasesensor 8 outputs the detected signal to the controller 1. ‘k’ inbrackets is a natural number and represents the k-th step of controlsampling. It is assumed that the symbol ‘k’ in brackets described lateralso represents the k-th step of control sampling.

The load machine 9 is an inertia load connected to the induction motor6. The load machine 9 is driven by the induction motor 6. The loadmachine 9 is, for example, a fan or blower.

The controller 1 controls the output power of the induction motor 6based on the phase currents Ias, Ibs, Ics and the detection signalsdetected by the phase sensor 8. Thus, the driving operation of theinduction motor 6 is controlled. The controller 1 employs, as a controlmethod, a dead-beat direct torque & flux control (DB-DTFC) method.

DB-DTFC is a scheme in which control is performed to make the torque andflux of the induction motor 6 directly reach target values in everycontrol period (every sampling period). Further, deadbeat control meansa method of performing sequential control to enable the target valuederived from the physical model of the induction motor 6 to be reachedin every control period. The controller 1 performs control to maximizethe loss so as to make regeneration energy be consumed during braking.When the loss model is used, control can be performed to minimize lossduring power running.

Further, DB-DTFC is a type of vector control. The complex vector usedfor this control is a vector represented by an orthogonal coordinatesystem wherein a d axis is orthogonal to a q axis. The d axis is animaginary axis, and the q axis is a real axis.

The controller 1 includes a speed/phase estimation module (observer) 11,a speed controller 12, a brake control switching module 13, a lossoperation amount calculation module 14, a DB-DTFC calculation module 15,two coordinate conversion modules 16 and 17 and a current/fluxestimation module (observer) 18.

The speed/phase estimation module 11 receives a detection signal fromthe phase sensor 8, and a calculated torque command value Tem*(k) fromthe brake control switching module 13. The torque command value Tem*(k)is an command value for air-gap torque. The speed/phase estimationmodule 11 calculates a rotor angle estimate value θr0(k) and a rotorangular velocity estimate value (speed feedback value) ωr0(k), based onthe detection signal from the phase sensor 8 and the torque commandvalue Tem*(k). The speed/phase estimation module 11 outputs thecalculated rotor angle estimate value θr0(k) to the current/fluxestimation module 18, and outputs the calculated rotor angular velocityestimate value ωr0(k) to the speed controller 12.

The speed controller 12 receives a rotor angular velocity command valueωr*(k), and also receives the calculated rotor angular velocity estimatevalue ωr0(k) from the speed/phase estimation module 11. The speedcontroller 12 calculates the torque command value Tem1*(k) to follow therotor angular velocity command value ωr*(k). For instance, the speedcontroller 12 performs proportional-plus-integral (PI) control. Thespeed controller 12 outputs the calculated torque command value Tem1*(k)to the brake control switching module 13.

The brake control switching module 13 receives the calculated torquecommand value Tem1*(k) from the speed controller 12, and receives apreset stator dq-axes flux command value λqds1*, a brake preparationflag Fg1 and a brake start flag Fg2. Based on the brake preparation flagFg1 and the brake start flag Fg2, the brake control switching module 13switches the torque command value Tem*(k) and the stator dq-axes fluxcommand value λqds*(k) to the values used during a normal operation anda braking operation, respectively. The brake control switching module 13outputs the torque command value Tem*(k) to the speed/phase estimationmodule 11 and the DB-DTFC calculation module 15, and outputs the statordq-axes flux command value λqds*(k) to the DB-DTFC calculation module15. The stator dq-axes flux command value λqds*(k) output from the brakecontrol switching module 13 is a scalar amount.

During a normal operation, the brake control switching module 13 outputsthe torque command value Tem*1(k), output from the speed controller 12,as a finally determined torque command value Tem*(k), and outputs apreset stator dq-axes flux command value λqds1* as the stator dq-axesflux command value λqds*(k). It should be noted that the stator dq-axesflux command value λqds*(k) used during the normal operation can be setto a desired value.

When braking is started, the controller 1 sets the brake preparationflag Fg1 to change the stator dq-axes flux command value λqds*(k) tomaximize loss. When the stator dq-axes flux has reached the value thatcauses maximum loss, the controller 1 sets the brake start flag Fg2,switches control from speed control to torque control, and performsbraking under torque control with the maximum torque command valueTem*(k) at which voltage increase in the DC link (capacitor 4) fallswithin an allowable range. The torque command value Tem*(k) and thestator dq-axes flux command value λqds*(k), at which maximum loss isobtained, are expressed by functions of speed. These speed functions arepreset in the loss operation amount calculation module 14 contained inthe brake control switching module 13. During braking, the lossoperation amount calculation module 14 changes the torque command valueTem*(k) and the stator dq-axes flux command value λqds*(k) in accordancewith the speed of the motor, using the speed functions.

The DB-DTFC calculation module 15 receives a calculated stator dq-axesflux estimate value λqds0(k) and a calculated rotor dq-axes fluxestimate value λqdr0(k) from the current/flux estimation module 18, andthe torque command value Tem*(k) and the stator dq-axes flux commandvalue λqds*(k) from the brake control switching module 13. The physicalmodel of the induction motor 6 is preset in the DB-DTFC calculationmodule 15. The physical model is obtained by replacing the inductionmotor 6 with an equivalent circuit and mathematizing the circuit.

Using the physical model, the DB-DTFC calculation module 15 performscalculation for executing DB-DTFC, based on the torque command valueTem*(k), the stator dq-axes flux command value λqds*(k), the statordq-axes flux estimate value λqds0(k) and the rotor dq-axes flux estimatevalue λqdr0(k). The DB-DTFC calculation module 15 calculates a torqueline Te(k+1) indicating a line for obtaining a desired torque in asubsequent control period. Namely, if the stator flux command value ison the torque line Te(k+1), a desired torque can be realized in thesubsequent control period.

Based on the torque line Te(k+1), the DB-DTFC calculation module 15determines a stator flux command value in view of the amount of loss inthe voltage-second coordinate system. The term “voltage-second” meansthe product of voltage and time (seconds), which represents flux. Sincethe amount of operation in DB-DTFC is flux, the output voltage of theinverter 5 is calculated by handling the flux as the product of thecontrol period, represented by “voltage-second,” and the output voltageof the inverter. During a braking operation, the DB-DTFC calculationmodule 15 determines a vector quantity of the volt-time (flux) so as tomaximize the amount of loss. Based on the determined vector quantity ofthe volt-time (flux), the DB-DTFC calculation module 15 calculates astator dq-axes voltage command value Vqds* expressed by the dq-axescoordinate system, using an command value for the output voltage of theinverter 5. The DB-DTFC calculation module 15 outputs the calculatedstator dq-axes voltage command value Vqds* to the coordinate conversionmodule 16. The DB-DTFC calculation module 15 also supplies thecurrent/flux estimation module 18 with the stator dq-axes voltagecommand value Vqds* as the stator dq-axes voltage Vqds(k) used by thecurrent/flux estimation module 18.

The coordinate conversion module 16 converts the stator dq-axes voltagecommand value Vqds* into three-phase stator voltage command values Vas*,Vbs* and Vcs*, and outputs the converted three-phase stator voltagecommand values Vas*, Vbs* and Vcs* to the inverter 5. Based on the inputthree-phase stator voltage command values Vas*, Vbs* and Vcs*, theinverter 5 outputs three-phase AC voltages.

The coordinate conversion module 17 receives three-phase stator currentsIas, Ibs and Ics detected by the current detectors 7 a, 7 b and 7 c andoutput from the inverter 5. Alternatively, the coordinate conversionmodule 17 may receive currents of two phases, and calculate the currentof the remaining one phase. The coordinate conversion module 17 convertsthe three-phase stator currents Ias, Ibs and Ics into a stator dq-axescurrent iqds(k) used in the calculation by the current/flux estimationmodule 18.

The current/flux estimation module 18 receives the rotor angle estimatevalue θr0(k) from the speed/phase estimation module 11, the statordq-axes voltage command value Vqds*(k) from the coordinate conversionmodule 17, and the stator dq-axes current iqds(k) from the coordinateconversion module 17. Based on the rotor angle estimate value θr0(k),the stator dq-axes voltage command value Vqds*(k) and the stator dq-axescurrent iqds(k), the current/flux estimation module 18 calculates thestator dq-axes flux estimate value λqds0(k) and the rotor dq-axes fluxestimate value λqdr0(k). The current/flux estimation module 18 suppliesthe DB-DTFC calculation module 15 with the calculated two flux estimatevalues λqds0(k) and λqdr0(k).

The control law of DB-DTFC will now be described.

FIG. 2 is a conceptual diagram according to the embodiment showing, in asingle voltage-second coordinate system, the torque line Te(k+1) derivedfrom the physical model of the induction motor 6, the range of a statorflux that can be output in a subsequent control period, and the lossmodel.

The control variables used for DB-DTFC are the torque command value(air-gap torque) Tem*(k) and the stator flux vector command valueλqds*(k).

It is supposed that the upper and lower arms of the power conversioncircuit of the inverter 5 are each formed of one switching element. Inthis case, the power conversion circuit is formed of six switchingelements. The six switching elements have eight switching patterns intotal. If these eight switching patterns are used, the voltage vectorsthat can be output from the inverter 5 form a hexagon as shown in FIG.2. Accordingly, the range of the voltages that can be output from theinverter 5 falls within the hexagon.

The torque line Te(k+1) is a line indicating a stator flux vectorλqds(k+1) for obtaining a desired torque in a subsequent control period.The torque line Te(k+1) is determined by the torque command valueTem*(k), the stator flux vector λqds(k) and the rotor flux vectorλqdr(k).

The loss is determined from the distance between the stator flux vectorλqds(k+1) and a loss model preset based on the circuit of the inductionmotor 6. Accordingly, by selecting the stator flux vector λqds(k+1) fromthe vectors on the torque line Te(k+1) in accordance with the lossmodel, the loss can be varied while a desired torque is being output.

FIG. 3 is a graph according to the embodiment analytically showing therange of the stator flux magnitude of the induction motor 6. Thevertical axis indicates the magnitude of the stator flux, the horizontalaxis indicates the speed of the rotor, and the depth indicates themagnitude of the torque.

In DB-DTFC, it is necessary to control the magnitude of the stator fluxwithin the range shown in FIG. 3. The magnitude range of the stator fluxis limited by voltage, current and loading condition. By the loadingcondition, the lower limit Lm3 of the magnitude of the stator flux isdetermined in accordance with the torque. When the speed of the rotor isrelatively low, the upper limit Lm1 of the magnitude of the stator fluxis limited by the rated current of the inverter 5 or the induction motor6. When the speed of the rotor exceeds a predetermined value (about 0.7p.u. in FIG. 3), the upper limit Lm2 of the magnitude of the stator fluxis limited in accordance with the induced voltage.

Further, by permitting the DC voltage of the inverter 5 to be variedwithin a certain range (e.g., 5%), the torque of the brake (this is anegative torque if the torque assumed during power running is defined tobe positive) can be strengthened, compared to the case where the DCvoltage of the inverter 5 is maintained constant.

Referring now to FIGS. 4 to 7, control by the controller 1 duringbraking will be described. FIGS. 4 to 7 show plots L1, L2 and L3 ofparameters during braking. The plots L1 show the case where variation(increase) in the voltage of the capacitor 4 is not allowed. The plotsL2 show the case where variation (increase) in the voltage of thecapacitor 4 is allowed within a range of 5%. Further, the plots L3 showthe case where the magnitude of the flux is maintained constant (at 1p.u.). This case is a comparative example to be compared with theembodiment. FIG. 4 shows the plots L1 to L3 of speed values. FIG. 5shows the plots L1 to L3 of flux command values. FIG. 6 shows the plotsL1 to L3 of torque values. FIG. 7 shows the plots L1 to L3 of DCvoltages. The plots L1 to L3 of the respective parameters shown in FIGS.5 to 7 indicate variations that have occurred in accordance with thevariation in speed shown in FIG. 4 and represented by the plots L1 toL3.

When the brake preparation flag Fg1 has been set to execute braking, thecontroller 1 increase the magnitude of the stator flux with the speedcontrol continued, in accordance with a preset stator flux commandvalue.

When the magnitude of the stator flux has reached the value that causesa maximum loss, the brake start flag Fg2 is set and the torque controlis executed so as to perform maximum braking within the upper limitrange of the DC linkage voltage in accordance with a preset torquecommand value. At this time, the flux and the torque are varied inaccordance with the speed of the induction motor 6. As a result, brakingis realized to reduce the speed. When the speed has reduced to a targetvalue, control is returned to speed control to reduce the magnitude ofthe flux to a rated value (1 p.u.).

The embodiment can also provide the following advantages:

Even if no new hardware is added, the loss of the induction motor 6 canbe increased during braking to enable to consume regeneration energy inthe induction motor 6 by applying the software to perform the DB-DTFC.

Thus, the regeneration energy can be consumed within the induction motor6 and the induction motor 6 can be decelerated in a shorter time.

Since the volt-time (i.e., flux) output from the inverter 5 iscontrolled and then supplied to the induction motor 6, the flux can bedirectly controlled, whereby the induction motor 6 can be operated tomaximize the loss.

By controlling the stator flux of the induction motor 6, the losseffective to consume regeneration energy can be quickly generated.Further, since in the embodiment, the flux can be directly controlled,highly responsive braking can be realized without undesirable torqueripple and without the influence of the first-order lag transferfunction upon the control.

It is to be noted that the present invention is not restricted to theforegoing embodiment, and constituent elements can be modified andchanged into shapes without departing from the scope of the invention atan embodying stage. Additionally, various inventions can be formed byappropriately combining a plurality of constituent elements disclosed inthe foregoing embodiment. For example, several constituent elements maybe eliminated from all constituent elements disclosed in the embodiment.

What is claimed is:
 1. A controller for a power converter configured todrive a motor, the controller comprising: a current detector configuredto detect a current output from the power converter; a flux vectorestimation module configured to estimate a flux vector of the motorbased on the current detected by the current detector; a torque linecalculation module configured to calculate a torque line in a physicalmodel resulting from mathemating a circuit equivalent to the motor basedon the flux vector estimated by the flux vector estimation module, thetorque line indicating a line for obtaining a desired torque in asubsequent control period; a stator flux command value calculationmodule configured to calculate a stator flux command value in accordancewith a loss; a voltage command value calculation module configured tocalculate a voltage command value based on the torque line calculated bythe torque line calculation module and the stator flux command valuecalculated by the stator flux command value calculation module; and anoutput voltage controller configured to control an output voltage of thepower converter based on the voltage command value calculated by thevoltage command value calculation module.
 2. The controller of claim 1,wherein the voltage command value calculation module calculates, inevery control period, the voltage command value for the stator flux toreach a target value derived from the physical model.
 3. The controllerof claim 1, wherein when a brake is put on the motor, the stator fluxcommand value calculation module maximizes the loss.
 4. The controllerof claim 3, wherein when the motor is in a power running operation, thestator flux command value calculation module minimizes the loss.
 5. Amotor drive system comprising: a motor; a power converter configured todrive the motor; and a controller configured to control the powerconverter, wherein the controller comprises: a current detectorconfigured to detect a current output from the power converter; a fluxvector estimation module configured to estimate a flux vector of themotor based on the current detected by the current detector; a torqueline calculation module configured to calculate a torque line in aphysical model resulting from mathemating a circuit equivalent to themotor based on the flux vector estimated by the flux vector estimationmodule, the torque line indicating a line for obtaining a desired torquein a subsequent control period; a stator flux command value calculationmodule configured to calculate a stator flux command value in accordancewith a loss; a voltage command value calculation module configured tocalculate a voltage command value based on the torque line calculated bythe torque line calculation module and the stator flux command valuecalculated by the stator flux command value calculation module; and anoutput voltage controller configured to control an output voltage of thepower converter based on the voltage command value calculated by thevoltage command value calculation module.
 6. The motor drive system ofclaim 5, wherein the voltage command value calculation modulecalculates, in every control period, the voltage command value for thestator flux to reach a target value derived from the physical model. 7.The motor drive system of claim 5, wherein when a brake is put on themotor, the stator flux command value calculation module maximizes theloss.
 8. The motor drive system of claim 7, wherein when the motor is ina power running operation, the stator flux command value calculationmodule minimizes the loss.
 9. A control method for a power converterconfigured to drive a motor, the control method comprising: detecting acurrent output from the power converter; estimating a flux vector of themotor based on the detected current; calculating a torque line in aphysical model resulting from mathemating a circuit equivalent to themotor based on the estimated flux vector, the torque line indicating aline for obtaining a desired torque in a subsequent control period;calculating a stator flux command value in accordance with a loss;calculating a voltage command value based on the calculated torque lineand the calculated stator flux command value; and controlling an outputvoltage of the power converter based on the calculated voltage commandvalue.
 10. The control method of claim 9, wherein in every controlperiod, the voltage command value is calculated for the stator flux toreach a target value derived from the physical model.
 11. The controlmethod of claim 9, wherein when a brake is put on the motor, the loss ismaximized.
 12. The control method of claim 11, wherein when the motor isin a power running operation, the loss is minimized.